TIMPZ: An Exquisite Building Block for Metal/Hydrogen Coordination Polymers

Article abstract of DOI:10.1002/ejic.201900229

The novel polynitrogenated ligand tetra-[4(5)-imidazoyl]pyrazine (TIMPZ, 1) and its pincer complex of Fe^II are described. This ligand has an outstanding ability to aggregate in 1D-superstructures by means of hydrogen bonding or metal chelation. TIMPZ shows an unprecedented conformational control over the supramolecular assembly. Metal or hydrogen coordination switches the conformation of peripheral rings driving the assembly preferences. For [Fe_n(TIMPZ)_n+1]²ⁿ⁺ complexes, UV/Vis spectroscopy shows a bathochromic shift of the main visible absorption band with higher “n” values. TIMPZ also has hydrogen-bond triggered excited-state intramolecular proton transfer (ESIPT) fluorescence, which is completely inhibited by chelation with transition metals.

Full text of DOI:10.1002/ejic.201900229

A Journal of
Accepted Article
Title: TIMPZ: An exquisite building block for metal/hydrogen
coordination polymers
Authors: Carolina Ligiéro, Thiago Valdares de Oliveira, Chárbel
Fontes, José Thiago Barragan, Fernanda So, Lauro Kubota,
René Nome, and Paulo Miranda
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201900229
Link to VoR: http://dx.doi.org/10.1002/ejic.201900229

10.1002/ejic.201900229
European Journal of Inorganic Chemistry
COMMUNICATION
TIMPZ: An exquisite building block for metal/hydrogen
coordination polymers
Carolina B. P. Ligiéro, Thiago V. Oliveira, Chárbel C. F. Fontes, José T. C. Barragan, Fernanda W. Y.
So, Lauro T. Kubota, René A. Nome, and Paulo C. M. L. Miranda*[a]
Abstract: The novel polynitrogenated ligand tetra-[4(5)-
imidazoyl]pyrazine (TIMPZ, 1) and its pincer complex of Fe(II) are
described. This ligand has an outstanding ability to aggregate in
1D-superstructures by means of hydrogen bonding or metal
chelation. TIMPZ shows an unprecedented conformational
control over the supramolecular assembly. Metal or hydrogen
To achieve a multimodal ligand capable of selectively self-
assembling by hydrogen bonding or by metal chelation to form
1D-superstructures depending on applied conditions, we
envisaged the construction of heterocycle bearing two different
conformations: the first favoring a linear array of hydrogen
bonds, and a second one having a preference for metal
chelation (TIMPZ, 1, Figure 1).
coordination switches the conformation of peripheral rings
2n+
driving the assembly preferences. For [Fen(TIMPZ)n+1
]
complexes, UV-Vis spectroscopy shows a bathochromic shift of
the main visible absorption band with higher “n” values. TIMPZ
also has hydrogen-bond triggered excited-state intramolecular
proton transfer (ESIPT) fluorescence, which is completely
inhibited by chelation with transition metals.
H
H
H
H
N
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1
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Introduction
H
H
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H
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H
N
Coordination polymers have caught the attention of the
scientific community due to their electronic, magnetic, and
optical properties.1 Structural queuing to form one-
dimensional supramolecular assemblies can lead to the
emergence of band-like electrical transport and tailored
chemical properties. Thus, cooperative interactions
between discrete molecules allow the appearance of
molecular-based ferromagnets, ferroelectric materials,
electrical conductors and non-linear optical materials
using coordination polymers.2 A major drawback for
coordination polymers involving highly charged cations is
the charge accumulation due to metal summoning.3 Many
different bridging ligands have been used to construct
such 1D superstructures, some of them bearing bipyridine
or terpyridine subunits.1-2 In this work, we introduce the
N
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n
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new ligand tetra-[4(5)-imidazoyl]pyrazine (TIMPZ,
1)
N
N
N
N
N
N
capable of promoting 1D assemblage. The presence of
imidazole nuclei avoids excessive charge accumulation in
heterocyclic nuclei due N-H polarization and allows higher
H
H
N
N
one-dimensional
elongation
of
the
aggregates.
N
N
Nevertheless, conformational change of peripheral
imidazole rings due to hydrogen or metal coordination
drives the assembly preferences allowing two different
superstructures.
Figure 1. Tetra-[4(5)imidazoyl]pyrazine (TIMPZ, 1) and conforma-
tion preferences for hydrogen bonding 1D-superstructures (right) and
1D-coordination polymers (left).
An extended array of hydrogen bonds could be formed in
neutral media due to imidazole ring association. Conversely, in
the presence of metallic cations it would be possible to change
the conformation of the imidazole rings to perform 1D-
coordination polymers into an octahedral environment.
[a]
Institute of Chemistry, University of Campinas, Josué de Castro, 10384-612,
São Paulo, Brazil.
E-mail: miranda@iqm.unicamp.br
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.201900229
This article is protected by copyright. All rights reserved.

10.1002/ejic.201900229
European Journal of Inorganic Chemistry
COMMUNICATION
N
N
N
N
N
N
N
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DMAS
O
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O
DMAS
DMAS
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DMAS
DMAS
H
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4
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1
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iv
i
iii
H
ii
N
N
HO
H
DMAS
2
3
Scheme 1. Reagents and conditions for preparation of TIMPZ (1): i) (a) NaH, acetonitrile, rt, 1.5h, (b) DMAS-Cl, acetonitrile, rt, 18h, 91%; ii) MeOH,
TEA, 3-benzyl-5-(2-hydroxyethyl)-4-methylthiazolium chloride, 50°C, 30min, 96%; iii) NH4OAc, pyridine, 80°C, 2h, 65%; iv) (a) HBraq, 100°C, 2h,
(b)
NH4OHaq,
88%.
In order to investigate this hypothesis TIMPZ (1) was
promptly prepared from 4(5)-imidazolecarbaldehyde (2) in 4
steps as described at Scheme 1.
intramolecular proton transfer (ESIPT) mechanism.5
Fluorescence was inhibited by strongly acidic or basic media,
as well as metal coordination.6 In the case of metal
coordination, ESIPT fluorescence suppression is due to
conformational exchange avoiding intramolecular hydrogen-
bonding (Figure 1) and consequently the proton transfer in the
excited-state.
Results and Discussion
DRX data clearly indicate that all rings in diprotonated
TIMPZ (A) are in a planar configuration owing to the formation
of a seven-membered ring. N···N distance was observed to be
as short as 2.543 Å and calculated N–H···N angle was about
159.7°. Similar chemical structures bearing strong
intramolecular H-bond qualitatively correlates with the more
DMAS protecting group proved to be essential to promote the
acyloin coupling and it was easily removed from the pyrazine
derivative 4 by diluted acid aqueous solutions or by heating.
The acyloin coupling was successfully promoted by thiazolium
carbene catalysts. Pyrazine nucleus was achieved by the
condensation of acyloin 3 with ammonium acetate in pyridine
at 80°C.4 Two different TIMPZ salts were crystallized
depending on the applied conditions: a diprotonated salt (A),
and a tetraprotonated salt (B) as shown in Figure 2.
downfield shift in 1H-NMR.6 Experimentally, the 
> 10
downfield shift and sharp N-H peak for weak acidic hydrogen
supports the intramolecular H-bond formation.6 Therefore, in
the case of the neutral TIMPZ, the N(2)–H···N(1) TIMPZ H-NMR
signals in THF-d8 located at  = 18.10 ppm suggests a low
barrier N–H···N hydrogen bond.7 This can also be evidenced by
the infrared frequencies of the N–H stretching observed at
~1000-600 cm-1.8
HRMS analyses of a diluted aqueous solution of TIMPZ
revealed the presence of oligomers bearing as many as 22
monomer units in four different degrees of protonation (Figure
3). We observed an exponential relationship between the
HRMS signal intensity and the number of monomer units in the
supramolecular structure for each protonation state. These
findings suggest a double hydrogen-bond superstructure by
means of resonance-assisted hydrogen bond (RAHB) in a
synergistic cooperation between hydrogen-bond strengthening
and -delocalization as shown in Figure 1.9 Increasing of
protonation at the supramolecular aggregate reinforces the
intramolecular N–H···N hydrogen bond due to RAHB
improvement. This phenomenon seems to be confined for each
monomer, which causes a slight lowering of intermolecular
N-H···N bonds.
Figure 2. ORTEP representation of (A) [(TIMPZ·2H)(PF6)2], and
(B) [(TIMPZ·4H)Br4·H2O]. Ellipsoids displayed at 50% probability.
Crystallographic data about compounds A and B were deposited in
Cambridge Crystallographic Data Centre with numbers CCDC-1868584
and CCDC-1583381, respectively.
We fitted the HRMS intensities with
a
linear
supramolecular model based on an array of monomer units in
which the binding energy between units is constant. The
supramolecular assembly is formed by hydrogen bonding
between adjacent TIMPZ molecules. As
a result, each
configuration is singly degenerate. Since the hydrogen bonding
pattern between adjacent TIMPZ molecules is assumed to
remain the same throughout the structure, the energy levels
Fluorescence spectra of TIMPZ shows a Stokes shift of
3854.5 cm–1, even in DMSO solution, and can be described by a
two-state system with pKa* = 2 showing an excited-state
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10.1002/ejic.201900229
European Journal of Inorganic Chemistry
COMMUNICATION
are equally spaced. Thus, the HRMS signal intensities are
associated with Boltzmann probabilities of finding the linear
supramolecular system microstate with a given length, and the
slope of each curve shown in Figure 4 directly gives the
average binding energy (in kBT units) between monomer units
for each group.
Figure 3. HRMS (ESI) of TIMPZ supramolecular arrays showing the coexistence four different degrees of protonation
(TIMPZn+H+, TIMPZn+2H+, TIMPZn+3H+, and TIMPZn+4H+). Ion intensity profile at diluted aqueous solution.
bonding strength. On the other hand, with increasing charges,
the length of the observed assemblies also increases, such that
the larger number of units is 5(+1), 11(+2), 16(+3) and 22(+4).
Thus, the interplay of electrostatics, hydrogen bonding, and
finite system size gives the observed results shown in Figure 4.
[Fe(TIMPZ)2](PF6)2 (6, Figure 5) was easily prepared by
addition of an aqueous solution of Fe(II) perchlorate into an
ethylene glycol solution of TIMPZ (1:2 ratio). Interestingly, the
same complex was obtained when Fe(III) was used as reagent,
even under ordinary conditions. In the last case, we detected a
complex mixture of oxidized byproducts of TIMPZ containing
quinonoid derivatives of the ligand. In both cases the
complexes were prepared in good to excellent yields (60-90%)
and they were silent into EPR analyses. They also furnished
identical HRMS, 1H and 13C-NMR, UV-Vis, CV, IR, and
conductimetry results. The signals in the UV-Vis absorbance
spectra (200-400 nm) correspond to the ligand charge transfer
Figure 4. Linearized plot of ion intensities for each protonated
state: TIMPZn+H+ (black), TIMPZn+2H+ (red), TIMPZn+3H+ (blue), and
TIMPZn+4H+ (green).
(LCT), ππ* transition. TIMPZ complex of Fe(II) has a strong
absorption at 569 nm (=1.156·104 L·cm-1·M-1) due to a metal-
to-ligand charge transfer (MLCT) which is red-shifted in
comparison to the analogous complexes where the bands is
Linear fit of the natural logarithm of HRMS intensities as a
function of number of monomers gave average binding
energies of -2.48(R2=0.94), -1.01(R2=0.98), -0.64(R2=0.98), and
-0.48(R2=0.91) kcal/mol respectively for TIMPZn+H+,
TIMPZn+2H+, TIMPZn+3H+, and TIMPZn+4H+, consistent with
weak (relative to the thermal energy) hydrogen bonding
between units, as expected. Since the sample size and thus the
number of monomer units is finite, the model also predicts that
the probability of finding the system with a given length is
inversely proportional to the polymer length (Figure 4).
In comparing binding energies, the larger negative slope for
+1 assemblies in relation to +2, +3, +4 aggregates means a
stronger decay for higher protonated assemblies (Figure 4).
Extrapolation of the retrieved binding energies as a function of
charge gives an intercept of -7.66 kcal/mol (R2=0.99) for the
neutral aggregate, a value consistent with medium hydrogen
observed at 350-450 nm.10
HRMS analyses of a diluted aqueous solution of 6 shown
the presence of the ion m/z 372.09167 (0.296 ppm error
considering [Fe(TIMPZ)2]2+ and a matched isotopic pattern to
the expected complex. We also detected small amounts of 1D
coordination polymers as large as 3 cations and 4 TIMPZ by
HRMS (ESI), even when an excess of the ligand was used. In
this case, the UV-Vis spectra of the aqueous solutions show
additional absorptions bands whose positions undergoes a
bathochromic shift upon increasing the length of the
coordination polymers. We observed max at 620 for n=2 and
720 nm for n=3, which are compatible to the metal-to-metal
plus ligand charge transfer (MMLCT) transitions.1,11
Cyclic voltammetry of 6 in glassy carbon, using Ag/AgCl
electrode as reference, shows only one quasi-reversible redox
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10.1002/ejic.201900229
European Journal of Inorganic Chemistry
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process. Observed anodic peak potential and its current were
respectively 0.516 V and 49 A. Conversely, observed cathodic
peak potential and its current were respectively 0.333 V and
21A. Observed conductivity of 6 in methanol (203 S·cm2·mol-
1) for a 1mmol·L-1 solution also indicated a 2:1 anion/cation
ratio.12
Figure 5. Structures of observed Fen(TIMPZ)n+1
]
2n+ complexes: [Fe(TIMPZ)2]2+ (left) and [Fe3(TIMPZ)4]6+ (right)..
The authors declare no conflict of interest.
Conclusion
In summary, starting from 4(5)-imidazolecarbaldehyde a new
bis N3 ligand, TIMPZ (1), was synthesized in 4 steps with 46%
global yield. TIMPZ has an outstanding ability to aggregate to
form 1D-superstructures and has a high affinity for several
metals. Both protonated and metal coordinated TIMPZ are able
to form 1D-supramolecular arrays as large as 22 units.
Additionally, an intriguing intramolecular low barrier
hydrogen bond (LBHB) was observed and it is associated to an
excited-state intramolecular proton transfer (ESIPT)
mechanisms which causes a Stokes shift of 3854.5 cm–1. The
fluorescence was observed even in DMSO solution, but it was
inhibited by strong acidic or basic media, as well as metal
coordination. [Fe(TIMPZ)2]2+ showed a quasi-reversible redox
process involving only one electron and a HOMO-LUMO energy
gap of 2.179 eV and a high molar absorptivity (11.555·103
L·cm-1·M-1) at 569 nm suggesting its application in
optoelectronic devices. The ability of TIMPZ to perform 1D
Keywords: coordination polymers · metal–organic
frameworks · hydrogen bonds · conformation driven
assembling · supramolecular chemistry
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We gratefully acknowledge financial support from the State of
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Conflict of interest
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European Journal of Inorganic Chemistry
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
The novel polynitrogenated ligand
TIMPZ shows an unprecedented
conformational control over the
supramolecular assembly triggered by
metal or hydrogen coordination.
Carolina B. Ligiéro, Thiago V. Oliveira,
Chárbel F. Fontes, José C. Barragan,
Fernanda W. Y. So, Lauro Kubota, René
A. Nome, and Paulo C. M. L. Miranda*
[Fen(TIMPZ)n+1]
2n+ pincer complexes
Page No. – Page No.
and 1D coordination polymers are
described and revealed bathochromic
shift of the main visible absorption
band with increasing of “n” values.
(TIMPZ)n 1D aggregates as larger as
22 units were detected and shown
ESIPT fluorescence.
TIMPZ: An exquisite building block
for metal/hydrogen coordination
polymers
Layout 2:
COMMUNICATION
Author(s), Corresponding Author(s)*
((Insert TOC Graphic here))
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Title
Text for Table of Contents
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